Weak link. The quench that has shut down the LHC occurred when a connection between magnets, like the one being welded here, melted.


Large Hadron Collider Down for at Least 2 Months

A faulty electrical connection apparently melted and caused last Friday's breakdown of the Large Hadron Collider (LHC), the world's new highest-energy particle smasher, officials at the European particle physics laboratory, CERN, near Geneva, Switzerland announced this weekend. Repairs will keep the machine off line for at least 2 months.

Researchers started circulating protons in the 27-kilometer long, $5.5 billion LHC on 10 September and had hoped to start smashing the particles together as early as this week. But the gargantuan machine literally sprung a leak last week when several of its main "dipole" magnets, which guide protons around the collider's two counter circulating rings, suddenly overheated in an event called a "quench" (ScienceNOW, 19 September). The violent temperature spike apparently ruptured the plumbing within the magnets that carries frigid liquid helium to them and keeps the magnets cooled to just 2 degrees above absolute zero.

Since then, CERN researchers have been trying to figure out precisely what caused the magnets to overheat. A superconducting magnet is essentially a coil of wire that generates a magnetic field when current flows through it. If kept extremely cold, the superconductor carries huge currents without resistance; a quench occurs when part of the superconductor overheats and acts like an ordinary wire. The resulting heat can trigger a run away reaction, toasting the rest of the magnet and converting the energy stored in its field to heat. Such an event can start if, for example, protons stray out of the beam pipe and into the surrounding magnet material.

Friday's quench occurred a different way, however. Within the LHC, the 15-meter-long, 35-metric-ton magnets are connected so that current from one magnet flows into the next. Without protons in the machine, researchers were ramping up the current in a chain of some 154 dipoles magnets when the superconducting connection between two magnets apparently overheated and melted, says CERN spokesperson James Gillies. In principle, that loss of current can set off surges of voltage and current that can quench the magnets.

Once the connection between magnets overheated, a whole battery of circuits in the "quench protection system" fired as expected to shunt current out of the magnets and, ironically, heat them to even out the toll from the quench. Nevertheless, the rupture of the helium line suggests that at least one $900,000 magnet was ruined. To make repairs, workers will have to warm an entire octant of the machine to room temperature, Gillies says, and the warming and recooling alone will take 2 months.

Researchers experienced similar problems when starting up their electron-proton collider, HERA, which ran from 1992 to 2007 at the German particle physics laboratory, DESY, in Hamburg, says spokesperson Reinhard Bacher. "When I remember back to the first year of HERA, we had three or four of these warm-up-and-cool-down cycles," he says. However, Bacher notes, because HERA was smaller, each cycle took weeks, not months.